The present invention relates in general to data storage systems. In particular, the present invention relates to a method and apparatus for estimating the flyheight of an airbearing slider in a storage device using variable spindle velocity and a washboard-sequence of washboard-sections provided on a surface of a storage disk.
A typical magnetic data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
The actuator assembly typically includes a coil assembly and a plurality of outwardly extending arms having flexible suspensions with one or more transducers and slider bodies being mounted on the suspensions. The suspensions are interleaved within the stack of rotating disks, typically using an arm assembly (E-block) mounted to the actuator assembly. The coil assembly, typically a voice coil motor (VCM), is also mounted to the actuator assembly diametrically opposite the actuator arms. The coil assembly generally interacts with a permanent magnet structure, and is responsive to a transducer positioning controller.
In a typical digital magnetic data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One of the information fields is typically designated for storing data, while other fields contain track and sector identification and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers which follow a given track and may move from track to track, typically under servo control of a position controller.
The head slider body is typically designed as an aerodynamic lifting body that lifts the transducer off the surface of the disk as the rate of spindle motor rotation increases, and causes the transducer to hover above the disk on an airbearing cushion produced by high speed disk rotation. The separation distance between the transducer and the disk, typically 0.1 microns or less, is commonly referred to as head-to-disk spacing or flyheight.
As disk storage devices become more sophisticated, flyheights are becoming smaller and smaller. Unfortunately, this trend of reduced flyheights increases the likelihood of catastrophic head-crash, in which the head makes physical contact with the disk surface and the resulting damage is sufficient to cause data loss. This problem is made worse when combined with the concurrent trends of higher areal recording densities and faster rotational spindle velocities.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field emanating from the magnetized locations on the disk.
Conventional data storage systems generally employ a closed-loop servo control system to move the actuator arms to position the read/write transducers to specified storage locations on the data storage disk. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read servo information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking).
A servo writing procedure is typically implemented to initially prerecord servo pattern information on the surface of one or more of the data storage disks. A servo writer assembly is typically used by manufacturers of data storage systems to facilitate the transfer of servo pattern data to one or more data storage disks during the manufacturing process.
In one known servo technique, embedded servo pattern information is written to the disk along segments extending in a direction generally outward from the center of the disk. The embedded servo pattern is thus formed between the data storing sectors of each track. It is noted that a servo sector typically contains a pattern of data, often termed a servo burst pattern, used to maintain alignment of the read/write transducers over the centerline of a track when reading and writing data to specified data sectors on the track. The servo information may also include sector and track identification codes which are used to identify the position of the transducer. The embedded servo technique offers significantly higher track densities than dedicated servo, in which servo information is taken from one dedicated disk surface, since the embedded servo information is more closely co-located with the targeted data information.
In a further effort to increase disk capacity, a proposed servo information format was developed, termed pre-embossed rigid magnetic (PERM) disk technology. As described and illustrated in Tanaka et al., Characterization of Magnetizing Process for Pre-Embossed Servo Pattern of Plastic Hard Disks, I.E.E.E. Transactions on Magnetics 4209 (Vol. 30, No. 2, November 1994), a PERM disk contains embossed servo information in a number of servo zones spaced radially about the disk. Each servo zone contains pre-embossed recesses and raised portions to form a fine pattern, clock mark, and address code. The fine pattern and address code are used to generate servo information signals. To generate these servo information signals, the magnetization direction of the raised portions and the recesses must be opposite. The magnetization process involves first magnetizing the entire disk in one direction using a high-field magnet. Then, a conventional write head is used to magnetize the raised areas in the opposite direction.
While use of a PERM disk may increase disk capacity, such an approach suffers from a number of shortcomings. Servo information is provided on a PERM servo disk in a two-step magnetization process, as described above. This significantly increases the amount of time required to write servo information to the disk. Moreover, during the second step of the process, servo information is not yet available on the disk. Thus, an external positioning system must be employed, thereby increasing the cost of the servo writing process. Additional concerns associated with PERM disk technology include durability.
Finally, the PERM disk, like other embedded servo techniques, still stores servo information in disk space that could otherwise be used for data storage. As a result, PERM disk technology, although still at the research level, has not been widely accepted by industry.
Pre-embossed rigid thermal (PERT) disk technology uses the thermal response of a magnetoresistive (MR) head induced by servo information on a storage medium in order to position the MR head. As described in U.S. Pat. No. 5,739,972, issued Apr. 14, 1998 to Gordon J. Smith et al. and assigned to the assignee of the instant application, a PERT disk includes servo information provided to induce a thermal response in the MR head. The servo information is typically provided in the form of pre-embossed surface profile variations on the disk. A controller controls the relative position between the MR head and the embossed disk track using the thermal response induced in the MR head.
Typically in PERT disk technology, a read signal from an MR head is filtered to separate thermal and magnetic components. As disclosed in U.S. Pat. No. 6,088,176, issued Jul. 11, 2000 to Gordon J. Smith et al. and assigned to the assignee of the instant application, the thermal and magnetic components of a MR read signal are separated using a finite impulse response (FIR) filter. The thermal component is the thermal response of the MR head to the surface profile variations on the PERT disk. For the purpose of track following, for example, the surface profile variations may include serrated inner diameter (ID) and outer diameter (OD) track edges, which are radially aligned. For each track, the ID edge serration has a different serration frequency than the OD edge serration. By examining the frequency content of the thermal component of the read signal, the off-track direction and magnitude of the MR head can be determined and an appropriate control signal provided to the actuator to position the MR head over the centerline of a track. This multiple-frequency track serration arrangement provides improved track following without sacrificing data capacity of a disk. Unlike embedded servo techniques, this arrangement does not store servo information in disk space that could otherwise be used for data storage.
Thus, higher areal density can be achieved in varying degrees through the use of technologies such as embedded servo, PERM and PERT. However, such technologies can exacerbate the problem of catastrophic head-crash. As mentioned above, higher areal density combined with trend of reduced flyheights can increase the likelihood of catastrophic head-crash. Further exacerbating this problem is the concurrent trend of faster rotational spindle velocities. The problem of catastrophic head-crash may be overcome by monitoring the flyheight of a slider. Unfortunately, known techniques of monitoring flyheight typically are difficult to implement, not sufficiently accurate and/or require a large amount of computation.
There exists in the data storage system manufacturing industry a need for an enhanced method and apparatus for estimating the flyheight of an airbearing slider in a storage device. The present invention addresses this and other needs.
In accordance with a first aspect of the present invention, there is provided a storage device having a storage disk, a transducer provided on a slider, an actuator provided to position the transducer relative to the storage disk, and a motor provided to rotate the storage disk relative to the transducer at a plurality of discrete storage disk velocities. The slider floats on an airbearing over the storage disk as the storage disk rotates. The storage disk includes a washboard-sequence having a plurality of washboard-sections each comprising surface profile variations having a pitch different than that of the surface profile variations of other washboard-sections. The pitches are selected from within a range of values likely to excite an airbearing resonance of the slider as the storage disk rotates at the plurality of discrete storage disk velocities.
The storage device may utilize the washboard-sequence to estimate the flyheight of the slider. For each of the washboard-sections and at each of the discrete storage disk velocities, a maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section is measured as the storage disk rotates at that storage disk velocity. A determination is made of a velocity/washboard-section combination that caused the largest measured flyheight modulation, i.e., the combination that provides an excitation frequency closest to the actual airbearing resonance frequency of the slider will produce the largest flyheight modulation. The result of this determination is used to estimate the flyheight of the slider. For example, the flyheight may be estimated using an empirical formula that relates the flyheight and the resonance frequency. Once the flyheight is estimated, a determination may be made of whether the flyheight is unsatisfactory, e.g., below a minimum flyheight or decreasing. Appropriate action may be taken when the flyheight is unsatisfactory, such as notifying the user and/or scrubbing the slider. Accordingly, catastrophic head-crash may be avoided and the useful life of the storage device extended.
Preferably, the storage device further comprises a peak-amplitude detector provided to measure, for each of the washboard-sections and at each of the discrete storage disk velocities, a maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section as the storage disk rotates at that storage disk velocity. The washboard-sequence preferably includes observation-sections each comprising a relatively smooth surface profile interlaced between the washboard-sections, the maximum amplitude of flyheight modulation resulting from each of the washboard-sections being measured by the peak-amplitude detector as the slider floats over an adjacent one of the observation-sections as the storage disk rotates at each of the plurality of discrete storage disk velocities.
The storage device preferably further comprises means for estimating the flyheight of the slider based on a combination of one of the discrete storage disk velocities and one of the washboard-sections that caused the largest flyheight modulation. Preferably, the storage device further comprises a memory provided to store, for each of the washboard-sections and at each of the discrete storage disk velocities, a disturbance-excitation frequency value and the measured maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section as the storage disk rotates at that storage disk velocity. In addition, the storage device preferably further comprises a logic circuit provided to determine the disturbance-excitation frequency that produced the largest measured maximum amplitude of flyheight modulation, and based on the determined disturbance-excitation frequency, whether the flyheight of the slider is below a minimum flyheight. The logic circuit preferably further compares the determined disturbance-excitation frequency and a previously determined disturbance-excitation frequency, and based on the comparison, determines whether the flyheight of the slider is decreasing.
In accordance with a second aspect of the present invention, there is provided a method of estimating the flyheight of an airbearing slider in a storage device having a transducer provided on a slider, a motor for rotating the storage disk relative to the transducer at a plurality of discrete storage disk velocities, and an actuator provided to position the transducer relative to the storage disk. The slider floats on an airbearing over the storage disk as the storage disk rotates. A washboard-sequence is provided having a plurality of washboard-sections each comprising surface profile variations having a pitch different than that of the surface profile variations of other washboard-sections. The pitches are selected from within a range of values likely to excite an airbearing resonance of the slider as the storage disk rotates at each of the plurality of discrete storage disk velocities. The storage disk is rotated at a first of the plurality of discrete storage disk velocities. For each of the washboard-sections, a maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section as the storage disk rotates at the first storage disk velocity is measured. The storage disk is then rotated at a second of the plurality of discrete storage disk velocities. For each of the washboard-sections, a maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section as the storage disk rotates at the second storage disk velocity is measured. A determination is made of a combination of one of the first and second storage disk velocities and one of the washboard-sections that caused the largest measured flyheight modulation, i.e., the combination that provides an excitation frequency closest to the actual airbearing resonance frequency of the slider will produce the largest flyheight modulation. The flyheight of the slider is estimated based on the determination step. For example, the flyheight may be estimated using an empirical formula that relates the flyheight and the resonance frequency. Once the flyheight is estimated, a determination may be made of whether the flyheight is unsatisfactory, e.g., below a minimum flyheight or decreasing. Appropriate action may be taken when the flyheight is unsatisfactory, such as notifying the user and/or scrubbing the slider. Accordingly, catastrophic head-crash may be avoided and the useful life of the storage device extended.
The estimating step preferably includes the steps of storing, for each of the washboard-sections and at each of the first and second storage disk velocities, a disturbance-excitation frequency value and a measured maximum amplitude of flyheight modulation of the slider resulting from the slider floating over that washboard-section as the storage disk rotates at that storage disk velocity, and determining the disturbance-excitation frequency that produced the largest measured maximum amplitude of flyheight modulation.
Preferably, the method further comprises the step of determining whether the flyheight of the slider is below a minimum flyheight based on the determined disturbance-excitation frequency. The method preferably further comprises the step of scrubbing the slider by reducing the storage disk velocity while the slider is floating over the washboard sequence, if it is determined that the flyheight of the slider is below the minimum flyheight. Preferably, the method further comprises the step of notifying a user if it is determined that the flyheight of the slider is below the minimum flyheight.
The method preferably further comprises the steps of comparing the determined disturbance-excitation frequency and a previously determined disturbance-excitation frequency, and determining whether the flyheight of the slider is decreasing based on the comparison step. The method preferably further comprises the step of scrubbing the slider by reducing the storage disk velocity while the slider is floating over the washboard sequence, if it is determined that the flyheight of the slider is decreasing. Preferably, the method further comprises the step of notifying a user if it is determined that the flyheight of the slider is decreasing.